In general, a plasma generator being a capacitive load apparatus that generates a large amount of ozone gas and a large amount of radical gas includes a plurality of discharge cells connected in parallel. Each discharge cell includes a pair of electrodes opposed to each other to form a discharge space with a dielectric located between the electrodes. In recent years, a very-large-scale plasma generator is in increasing demand which includes laminations or blocks of a plurality of discharge cells connected in parallel. With a source gas being supplied in the discharge space in the plasma generator, a power supply apparatus applies an alternating-current high voltage between the discharge cells. The gas in the discharge space is excited by an electric field caused by the application of the voltage, generating a large amount of ozone gas and a large amount of radical gas.
The generated ozone gas and the generated radical gas often find use as a film deposition gas for a functional film such as an oxide insulating film or as a cleaning gas for components mainly in the semiconductor manufacturing field, the solar photovoltaic panel manufacturing field, and the flat display manufacturing field. In these fields which requires the ozone gas and the radical gas, these gases need to be supplied in large quantities and need to be supplied stably at high concentrations and high purities on a 24-hour basis while the amount and the concentrations of these gases that are generated and output need to be controlled stably and easily.
In general, loads driven due to the application of alternating-current voltage includes, besides a resistive (R) load such as a thermoelectric apparatus, an inductive (L) load such as a motor load and a capacitive (C) load associated with apparatuses that accumulate electric charges and apply a high voltage. Apart from the resistive (R) load such as the thermoelectric apparatuses, the inductive (L) load such as the motor load generally has constant impedance, and accordingly the electric power input increases in proportion to the increasing rate of voltage supplied from the power source to the load. Thus, the inductive load is relatively stable. In contrast, the capacitive load apparatus (C) such as a plasma generator is a nonlinear load that has inconstant impedance, which varies depending on the load conditions. Thus, it is very difficult to stably operate the plasma generator by supplying a voltage from the power supply apparatus. This is more likely to cause the breakage and the like of the discharge cell portion in the conventional plasma generator. It is therefore difficult to stably operate the plasma generator for a long period of time through the use of the voltage from the power supply apparatus.
The application of alternating current to a load being an inductive load or a capacitive load causes a phase lag or a phase lead of a load current Id relative to an applied load voltage Vd. Consequently, the ratio (load power factor ηd=PW/PQ) of an actually supplied electric power capacity PQ (=Vd×Id) to an active power PW supplied to the load becomes extremely small. Increasing the active power PW of a power supply apparatus having a small load power factor ηd requires the greater electric power capacity PQ (=Vd×Id), and thus a very-large-scale power supply apparatus needs to be installed.
For a smaller power supply apparatus, a power factor improvement apparatus (power factor improvement means) has been known which is mounted on the output unit of the power supply apparatus for improvement of the power factor ηd. The different power factor improvement apparatuses are provided for an inductive load and a capacitive load. The power factor improvement apparatus for the inductive load is a capacitor bank, which is provided to improve the L load. The power factor improvement apparatus for the capacitive load is a reactor, which is provided to improve the C load. The power supply apparatus works through the use of the inductive load (or the capacitive load) and the power factor improvement apparatus at around an alternating-current voltage frequency fc (resonance frequency) that creates the resonance state between the load and the power supply apparatus.
The resonance frequency fc is given by fc=½·π·(L·C)0.5 (hereinafter referred to as Expression (1)).
For the inductive load, substituting the capacitor bank being the power factor improvement apparatus into C of Expression (1) yields the resonance frequency fc. For the capacitive load, substituting the reactor Lp being the power factor improvement apparatus into L of Expression (1) yields the resonance frequency fc. The power supply apparatus works on a frequency range associated with the resonance frequency fc, whereby the power factor of the power supply apparatus is improved.
Patent Documents 1 to 3 are examples of the prior techniques for improving the power factor of the power supply apparatus that applies alternating-current electric power to the plasma generator being the capacitive load.
The power supply apparatus for alternating-current load disclosed in Patent Document 1 includes a transformer (inductor) for improving the power factor which is provided for a discharge load (discharge cell) that generates plasma.
The power supply apparatus for alternating-current load disclosed in Patent Document 2 includes a transformer (inductor) for improving the power factor which is provided for a discharge load (discharge cells) that generates plasma. Patent Document 2 discloses that the inverter circuit unit having the frequency control function allows for the power factor in the inverter output unit to be optimally controlled in the region in which greater electric power is input to the load. The power supply apparatus according to Patent Document 2 performs the frequency control to allow proper operation, during the occurrence of failure in one or some of the discharge cells, through the use of the remaining discharge cells.
According to the technique in Patent Document 3, in the plasma generator including a plurality of discharge cells, load de-energization fuses are provided for the respective discharge cells. According to the technique in Patent Document 3, any failure in a discharge cell causes a fuse provided correspondently to the discharge cell to burn out, thus interrupting the electric power supplied to the discharge cell. Further, Patent Document 3 discloses the power supply apparatus for three-phase alternating-current loads and the system for improving the power factor by operating the power supply apparatus at a predetermined frequency around the resonance through the use of the load capacitance value and reactors, the predetermined frequency being fixed on the power supply apparatus side.
The respective power supply apparatuses according to the above-mentioned patent documents include, for the improvement of power factor, inductive inductances (reactors) each located between the output side of the power supply apparatus and the plasma generator.